Here's how to do it in iTunes:
1. Create a new Smart Playlist with the following rule - "Size is greater than 0 MB" This will have the affect of adding every item in your music library to this smart playlist.
2. Confirm that your "Preferences: Advanced: Burning" is set to "Data CD"
3. Select your Smart Playlist (name it backup, if you'd like). Confirm that it indeed does have the same number of songs and data size as your library.
4. Click the "Burn Disc" button in the top right hand corner of the iTunes window.
5. Stick a DVD/CD into your burner. I can't remember exactly if iTunes tells you ahead of time how many discs (of the size you stuck in) you'll need, but it does say something to the effect that it will take multiple discs, blah blah blah.
6. Wait, Insert next disc, wait, repeat...
After you've created your full backup set, it's probably not something you'll want to repeat all that often - but here's how you can easily make incremental backups as you need them:
7. Edit your Backup Smart Playlist - change the "File Size is greater than 0MB" to "Date Added is after <today's date/date of your backup>" - now the playlist will show all files added to your library after your full backup.
I usually wait until this playlist becomes DVD-sized, and then burn it to disc. Then I change the date in the rule to the current date, and it's set up for the next time around.
The last time I did a full backup to DVD, my library was 35GB and it took 8 DVDs and at least a couple of hours - at 8X. Now that my library has grown to 90GB (finally digitized all of my music, and now use AAC192 or 256 instead of 128) I have to keep it on it's own external hard drive (which also holds other music/video/photo stuff that's too big to keep on my PowerBook). With HD prices so cheap these days, I purchased another hard drive and external case and use it to backup both my PowerBook's drive and my external 'media drive.'
I know that this sort of mirroring is not a true backup in the strictest sense, but between that and archiving projects and data to DVDs as appropriate (including my iTMS purchased music) I feel that this is the right mix of data security and ease of use for me... after all, even the best backup plan is useless if it's too much of a pain to do regularly.